This invention is in the field of welding particularly as applied to welding of small diameter wires on pins for electronic circuitry. Many of the principles of such welding and mechanical apparatus for performing the welding operation are set forth in U.S. patent application Ser. No. 857,258 now U.S. Pat. No. 3,627,970 entitled "Method for Welding a Conductor Through Cold Flowable Insulation" by John F. Weatherman, Gary J. Walker and Larry R. Conley.
In many types of modern electronic circuitry, paraticularly those using integrated circuits, a non-conductive substrate or board is used for mounting the electronic circuit components. Such a board may have a large number of metal pins extending through the board for making electrical connections. Typically integrated circuit packages are connected by conventional means to the pins on one side of the board. On the opposite side the pins may be interconnected by wires welded directly to the ends of the pins. In such an arrangement a fine metal wire sheathed in plastic insulation is used. The plastic is "cold flowable" so that it plastically flows at room temperature when under stress.
The cold flow characteristics of the insulation are taken advantage of in the welding operation to avoid the necessity of mechanically stripping the insulation from the wire at the point where welding is desired. Typically the welding is done in a welding machine having a table or the like for supporting the circuit board. A lower electrode extends through an aperture in the table for engaging the lower end of a pin. An upper electrode is typically vertically movable for retraction away from the board and for contact with the end of the pin during a welding cycle. During operation a fine insulated wire is brought down through the hollow upper electrode and passes laterally at its lower end so that when the electrode is pressed downwardly the wire is pinched between the peripheral lip at the tip of the electrode and the end of the pin. Pressure on the electrode causes the plastic insulation to flow, bringing the wire into electrical contact with the electrode and the pin.
The pressure required to break through the plastic insulation is too high for welding since the heated wire would not have sufficient strength and could be penetrated by the electrode. The pressure is therefore reduced to an optimum pressure for assuring good welding between the wire and the pin, and then a welding current is applied for a short time. As soon as the weld is made the pressure is relieved, the electrode retracted and the board translated to a new position with another pin between the electrodes. As the board is moved, wire is drawn through the hollow upper electrode for welding to the next pin so that wire can be "stitched" from pin to pin in any desired pattern.
Prior apparatus for such welding has employed a substantially fixed lower electrode approximately flush with the table that supports the circuit board for making contact to the lower end of the pins. The upper electrode is mounted on a support structure that can slide vertically in the apparatus. This electrode is moved and pressure is applied by the upper electrode to the wire as the movable member is forced into position by a complex mechanism of cams, levers and springs.
Such a mechanism is subject to significant limitations for production line operations because of the inherent inertia of the relatively massive parts and the rigidity of the parts which makes them substantially unyielding. In order to counter such limitations it is necessary that the pins on a circuit board be of substantially uniform length and be uniformly positioned relative to the boards so that good contact is obtained with the lower electrode, and the desired pressures are applied by the upper electrode. In addition to being relatively unyielding, the high inertia of the relatively massive parts in the mechanical arrangements forces longer cycle times than desirable. Even though the entire welding cycle may be only in the order of seconds it should be recognized that thousands of such cycles may be covered in a day's time and relatively small time intervals accumulate as substantial cost elements.
It is, therefore, desirable to provide a welding apparatus having compliant electrodes for accommodating variations in pin position or length and for yielding during the welding operation. It is also desirable to provide a welding machine having a faster cycle time than has heretofore been available.